Consider the follow content of "WorksOn" relation in the company database:
The relation "Emp_Proj" is obtained by:
Emp_Proj ÷ emp4_proj = {SSN1, SSN4} 
The reason that SSN1 is part of the result is all the tuples (SSN1, P1), (SSN1, P2), (SSN1, P3) are in the original data set
The reason that SSN4 is part of the result is all the tuples (SSN4, P1), (SSN4, P2), (SSN4, P3) are in the original data set

i.e., suppose "John Smith" works on projects {P1, P2, P3}, we must find employees that also work on P1, P2 and P3 (the employee may work on other projects in additions to P1, P2 and P3)
Solution:
Remember how the set division works:
Employees SSN1, SSN4 works on all projects P1, P2 and P3

i.e., suppose department #5 controls projects {P1, P2, P3}, we must find employees that also work on P1, P2 and P3 (the employee may work on other projects in additions to P1, P2 and P3)
Strategy:

Solution: